This invention concerns ball screw mechanisms of the well known type using a series of bearing balls interposed between aligned female thread forms in screw and nut members. The rolling ball interengagement greatly reduces the friction when the nut or screw is driven such that these devices are quite efficient and in widespread use in a variety of applications.
In U.S. Pat. No. 5,295,406 there is described a helical spring element inserted at one end of the nut beyond the point where the balls are present, occupying additional female thread turns. The presence of the helical spring element prevents relative axial movement between the screw and nut even if the balls are lost.
However, it is a cost disadvantage to assemble an additional component into the screw and nut threads. The spring element also may possibly engage the threads to create a frictional drag if close tolerances are not held, which would lessen the efficiency of the ball screw unit.
U.S. Pat. Nos. 6,401,557 B1 and 2,447,439 also describe ball screw mechanisms incorporating a separate nut element having male threads in the nut to prevent thrust forces from being transmitted back to the driving mechanism or motor. However, the male threads also comprise a separate element which is assembled into the nut, and are also normally engaged to create a frictional drag.
It is the object of the present invention to provide a ball screw mechanism which has an opposing thread feature preventing substantial relative axial travel of the screw and nut but which does not require an additional component to be assembled into the mechanism nor introduces appreciable frictional drag during normal operation of the ball screw.